Your grandmother says it's because you watch too much TV. Your teacher thinks it's because you're bored. And that creepy guy on the bus just slapped you with a phonebook in an attempt to exorcise the devil living in your molars. All because you're yawning.

There's got to be a better explanation, right?

Scientists haven't nailed down the exact cause of our 240,000 lifetime yawns yet, but there's good support for the yawn as a temperature regulator. In 2007, researchers at the University of Albany showed students a video of people yawning. Half the students were instructed to breathe through their nose, and the other half were told to in and exhale through their mouths. Of the mouth-breathing group, half of them yawned at the video, but among nose breathers, yawning was pretty much non-existent.

Huh?

It could be that the mouth-breathers were overheating their gray matter. Because the brain burns up to a third of our daily caloric intake, it generates a bit of heat. And our brains work better when they're not too toasty.

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To help keep the temperature down, blood vessels in the nasal cavity and face send cool blood to the brain. The thought is that breathing through the mouth doesn't allow the brain to be as efficiently cooled. When you yawn, it causes the expansion and contraction of the maxillary sinus, a cavity located in the cheek. The sinus shoots air upward, kicking cool air at our headspace.

In another test, the Albany researchers placed a cold pack, a room-temperature pack, or a warm pack onto each participant's foreheads. Those with the frosty domes yawned less than those in the other two groups. And last year researchers at Princeton found that we're less likely to yawn when the temperature outside exceeds our internal body temperature, supporting the theory that yawning has a thermoregulatory function. That 112 degree summer-in-Arizona air is bad for your mind, man!

But if yawning just keeps our brain sufficiently chilled, why is yawning the most contagious condition since the Black Death?

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There's some research suggesting that yawning can hint at emotions ranging from interest and stress, to, um, the desire to get it on. The yawn as an indicator of arousal came up as a theory at the first annual International Conference of Yawning (yup), held in Paris (yup) in 2010, when a chasmologist (someone who studies yawning (yup)) pointed out that sexologists often get patients complaining about yawning in the lead up to—and during—sex.

The yawn ripple effect could also be an evolved response, with the function of keeping groups alert when it counts. Hanging out in bear country? If yawning is your body's way of increasing brain function, passing on a cool brain boost to the clan would be a smart move when you all need to be on the look out for danger.

While a whole host of animals yawn—including snakes and fish—not all of them are susceptible to the contagious yawn. Chimpanzees experience the domino effect. And dogs cannot only catch a yawn from another pup, but from a person, too. Researchers think it might signal a social connection in animals, and in humans, possibly empathy. There's research showing that friends and loved ones catch each other's yawns more readily than they do from outsiders.

For me, whether it's the time change or just an overheated brain, writing this article has sent me into yawn overdrive. Even the word has had ((YAWN!)) a repeated effect on me. Brb. Next week.

Giz Explains is where we break down whatever science or tech questions are scratching at the backs of our noggins. Got questions of your own? Shoot us email at explains@gizmodo.com with "Explain this!" in the subject line, and we'll see about answering.